[LLVMdev] Functions: sret and readnone

[Stephan]

Duncan, thanks for your answer!

> In order to perform this transform the optimizers would have to work out
> that sample does not modify any global state.  This cannot be done without
> knowing the definition of sample, but you only provide a declaration.

Which is why I am trying to supply this additional information in a
custom alias analysis pass, but it doesn't seem to work. (The
AAEvalPass stats are precisely for this custom pass.)

Could you take a look at the code, please? Am I missing something
here?


class VISIBILITY_HIDDEN MySretAliasAnalysis : public FunctionPass,
public AliasAnalysis
{
	std::map<std::string, bool> _srets;

public:
	static char ID;

	MySretAliasAnalysis() : FunctionPass(&ID)
	{
		_srets["sample$int$float2"] = true;
		_srets["sample$int$float3"] = true;
	}

	void getAnalysisUsage(llvm::AnalysisUsage &usage) const
	{
		AliasAnalysis::getAnalysisUsage(usage);
		usage.setPreservesAll();
	}

	bool runOnFunction(Function &F)
	{
		AliasAnalysis::InitializeAliasAnalysis(this);
		return false;
	}

	ModRefBehavior getModRefBehavior(CallSite CS,
std::vector<PointerAccessInfo> *Info = 0)
	{
		if(_srets.find(CS.getCalledFunction()->getName()) != _srets.end())
			return AliasAnalysis::AccessesArguments; // only accesses args, no
globals
		return AliasAnalysis::getModRefBehavior(CS, Info);
	}

	ModRefBehavior getModRefBehavior(Function *F,
std::vector<PointerAccessInfo> *Info = 0)
	{
		if(_srets.find(F->getName()) != _srets.end())
			return AliasAnalysis::AccessesArguments; // only accesses args, no
globals
		return AliasAnalysis::getModRefBehavior(F, Info);
	}

	ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size)
	{
		std::string functionName = CS.getCalledFunction()->getNameStr();
		if(_srets.find(functionName) != _srets.end())
		{
			if(CS.hasArgument(P))
			{
				if(CS.getArgument(0) == P)
					return AliasAnalysis::Mod; // modify value pointed to by sret
param
				else
					return AliasAnalysis::NoModRef; // there aren't any other pointer
args
			}
		}
		return AliasAnalysis::getModRefInfo(CS, P, Size);
	}

	bool hasNoModRefInfoForCalls() const { return false; }
};


> If you provided a body too then the GlobalsModRef analysis might be able to
> work it out.

That's not an option because I want the sample function to be resolved
as an external function by the jitter.

Thanks for your time,
Stephan